Laser interferometers are widely used to measure distances or displacements. One type, polarization interferometers, operate by projecting a coherent beam to a polarizing beamsplitter arranged to provide two coherent orthogonally polarized beams. One of the orthogonally polarized beams is projected along the fixed-length reference path of the interferometer and one along the distance-measuring object or sample path. The beams are directed through respective quarter wave plates, and reflected from the reference surface and the sample surface, respectively. The reflected beams return through the quarter wave plates and are recombined at the same polarizing beamsplitter, to form an output beam having two orthogonally polarized components. The desired distance or displacement measuring information is carried by the phase difference between the orthogonally polarized components of the output beam.
Numerous schemes have been devised for measuring this phase difference. In one common scheme, a quadrature detector is used. Various types of quadrature detectors are known. Such quadrature detectors typically include multiple beam paths, beamsplitters, waveplates, polarizers, and multiple detectors. Differences between the components and alignment of the multiple beam paths contribute to differential phase and amplitude errors and/or environmental sensitivity and changes over time in the resulting quadrature signals. The errors associated with these differences may be small, and they are frequently overlooked or ignored. Thus, the accuracy of the resulting phase measurement may be limited by the quadrature detector errors. The present invention is directed to providing an apparatus that overcomes the foregoing and other disadvantages.